I am looking at an installation of 3 - 150 watt High Pressure Sodium light fixtures on a 20 amp/120 volt circuit. I would like to use one common 1800 watt (ballast rated) photocell for all three lights. My question is: To determine the load on the photocell is it as simple as adding the total lamp wattage on the circuit or are there other factors to take into consideration.